Be ready for European brown rot in tart cherries in 2014

The potential for European brown rot infection is high this season in orchards with carryover shoot strikes from last year. Applications of Indar will be required for effective control if we experience cold, wet weather.

European brown rot symptoms on Montmorency tart cherries. Arrows point to infected spurs from the previous season that serve as overwintering sites of the pathogen and sites of sporulation in the current season.

European brown rot symptoms on Montmorency tart cherries. Arrows point to infected spurs from the previous season that serve as overwintering sites of the pathogen and sites of sporulation in the current season.

European brown rot is a disease of tart cherry caused by the fungus Monilinia laxa. The main symptoms of European brown rot are blossom blast followed by shoot dieback (see photo). European brown rot symptoms are typically severe on Balaton and much less common and sporadic on Montmorency. However, 2013 was an exceptional year in that European brown rot infection was heavy in some Montmorency blocks, particularly those subjected to slow-drying conditions following wetting events that occurred at the white bud (popcorn) stage.

The occurrence of shoot dieback symptoms of European brown rot occur within two to three weeks after flower infection, after which the pathogen M. laxa remains quiescent in the infected shoots for the rest of the season and through winter. The fungus will become active again as the trees break dormancy and sporulation occurs prior to and through bloom. From observations made last year, as few as three to five infected shoots from the previous season can produce enough spores to infect most of the flower clusters on a tree.

European brown rot infection is favored by colder weather prior to and during bloom (high temperatures in the 30s to 40s) with extended wetting events – rain followed by at least 18-24 hours with relative humidity above 90 percent. Infection potential is also affected by cultivar; one possible reason for the susceptibility of Balaton is that M. laxa infects Balaton flower tissue more easily and thus does not require as long of a wetting period for infection.

Growers need to be aware of and ready for European brown rot infection in 2014, particularly in blocks with significant European brown rot infection last year. Sites affected by fog or with reduced air drainage and longer drying conditions are also at risk.

Fungicide sprays targeting European brown rot must be initiated at the white bud stage prior to a rain event. A second application follows usually one week later during bloom. Fungicide application, particularly on Montmorency, can be delayed if we are experiencing prolonged dry weather during bloom. There is currently only one fungicide available with confirmed efficacy against European brown rot and that is Indar 2F, used at a rate of 6 fluid ounces per acre. While there is a Section 24(c) label available allowing the use of up to 12 fluid ounces per acre per Indar application, the lower rate should be used at this time as our current analyses indicate that M. laxa isolates are sensitive to sterol inhibitor fungicides.

Note also that Michigan State University Extension researchers will be testing six different fungicide modes of action in 2014 for European brown rot control on Balaton with a hope of identifying alternatives to Indar that can be used to target European brown rot in 2015 and beyond.

Dr. Sundin’s work is funded in part by MSU‘s AgBioResearch.

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